This application is a 371 of PCT/CZ99/00033 filed Oct. 4, 1999.
This invention relates to anticorrosive plastic packaging materials comprising synergistic mixture of contact-vapour phase corrosion inhibitors, which are suitable for protection of iron, aluminum and alloys of these metals against corrosion.
Corrosion of articles made of iron, aluminum and alloys of these metals, non-ferrous metals etc. causes their functional and aesthetic devaluation. To diminsh or completely rule out these negative phenomena number of methods and means of protection against corrosion are employed, out of them the use of protective packaging materials containing metal corrosion inhibitors, predominantly plastics films, foams, containers etc. are the most important ones.
Most of the known plastic packaging materials comprising contact and/or vapour phase corrosion inhibitors exhibit sufficient inhibition efficiency but only for short term protection of steel surfaces (U.S. Pat. No. 3,967,926-1976, U.S. Pat. No. 4,290,912-1981). This is predominantly determined by the fact that the basic inhibiting component is, according to the above references, anion of nitrous acid or primary amine which are effective only in case of ferrous metals and the inhibition component is incorporated into the packaging material on a porous carrier. Moreover the prevailing part of known plastic packaging materials containing contact and/or vapour phase corrosion inhibitors comprise salts of nitrous acid (U.S. Pat. No. 5,332,523), particularly dicyclohexylamine nitrite (U.S. Pat. No. 5,422,187-1995, patent JP 63,210,285-1988, etc.) or organic salts of chromic acid, particularly cyclohexylamine chromate and dicyclohexylamine chromate (U.S. Pat. No. 4,275,835-1981), which can be industrially employed only to certain extent because of their missing hygienic approval. CS AO 223373 claims anticorrosion material containing mixture of inorganic benzoates with benzotriazole. The JP-A-59,023,884 describes a plastic anticorrosive material comprising a salt of benzotriazole or its derivatives and organic amine possibly complemented by a salt of a benzoic acid, its derivatives, or a fatty acid and an organic amine. Use of a sodium nitrite, sodium benzoate and benzotriazole is disclosed in FR-A-1508668 but for a paper impregnation.
When silica gel appears in some anticorrosion systems, it is usually employed as a desiccant or as a carrier of some anticorrosion inhibitors particularly of anhydrous molybdates (e.g. patents U.S. Pat. No. 5,332,525, U.S. Pat. No. 5,320,778, U.S. Pat. No. 5,209,869, U.S. Pat. No. 5,393,457). In U.S. Pat. No. 5,393,457 a plastic anticorrosive packaging material comprises besides anhydrous molybdate and silica gel also sodium nitrite and benzotriazole.
The effectiveness of contact-vapour phase inhibitors is determined not only by their suitable chemical structure but also by their vapour tension at the application temperature. To act as anticorrosive inhibitors they have to be at first evaporated and consequently condensed on the surface of the metal article to be protected. The protecting layer is very thin, even only monomolecular, therefore sufficient inhibitor vapour tension is usually in the range of 1,33xc3x9710xe2x88x921 to 1,33xc3x9710xe2x88x921 Pa at room temperature.
Anticorrosion inhibitors were originally used only in connection with anticorrosive packaging materials based on paper which was simply soaked with inhibitor solutions practically at room temperature, so the selection of suitable compounds was less limited.
This is different in case of plastic packaging materials, e.g. low density polyethylene film which is processed at temperatures above 160xc2x0 C. Inhibitors have to be added to the material destined for production of anticorrosive plastic packaging material prior to its processing and therefore they are exposed to relatively high processing temperature and consequently partially lost either by evaporation or sublimation. Not only the loss of inhibitor but also emissions released cause difficulties during production and moreover increase also production cost. Quite a few inhibitors are not sufficiently compatible with particular plastic packaging material and exude to its surface. If the migration is too fast, the loss of inhibitor from the packaging material is also too fast and the inherent protection period shortens. Too fast inhibitors exudation also shortens storing period of finished packaging products or semi-finished articles destined for their production, worsen surface appearance and touch of packaging products.
We have observed that the system of the contact-vapour phase inhibitors comprising salts of benzoic acid and/or nitrous acid, 1,3-benzodiazole C7H6N2 and/or its 1-methyl derivative, 1H-benzotriazole and/or its methyl derivative of the general formula 
if it is combined with suitable grades of amorphous silicium oxide forms a synergistic mixture exhibiting higher anticorrosive protection, decreases formation of emissions during packaging article production, limits exudate formation on the packaging product surface and thus improves its appearance.
The mechanism of the observed synergy, as it is evident from the patent examples, is explained by the interaction of N-containing inhibitors with the surface of a selected silicium oxide grade.
The nature of anticorrosive plastics packaging materials suitable for protecting iron, aluminum and alloys of these metals against corrosion is based on the incorporation of the synergistic mixture of the contact-vapour phase inhibitors comprising 0,01-2,0 wt. % of the salt or the mixture of salts of benzoic acid and/or nitrous acid and 0,001-1,5 wt. % of 1,3-benzodiazole C7H6N2 and/or its 1-methyl derivative and 0,001-1,0 wt. % of 1H-benzotriazole C6H5N3 and/or its methyl derivative of the general formula 
and 0,01-4,0 wt. % of amorphous silicium oxide SiO2.
By the salt or mixture of salts of benzoic acid and/or nitrous acid is according to this invention understood the salts or the mixture of salts of alkali metals, the salts of alkaline-earth metals and/or the salts or the mixture of ammonium salts.
By the plastic packaging product it is according to this invention understood film or packaging products made out of the film (sacks, bags, etc.), then injection or blow molded packaging products such as bottles, boxes, containers etc., made out of the anticorrosive plastic packaging material.
By the plastic packaging material it is according to this invention understood polyethylene and copolymers of ethylene with higher alpha olefins in the density range of 860-967 kg/m3, copolymers of ethylene comprising 0,5-40 wt. % of vinylacetate or C1-C4 alkylesters of acrylic or methacrylic acid, polypropylene, copolymers of propylene with ethylene and/or with higher C4-C8 alpha olefines having comonomer content 0,1-10 wt. %. The plastic packaging material may also be composed of the mixture of hereinbefore stated polymers.
The salts or the mixture of salts of benzoic and/or nitrous acid which do not melt at the polymer processing temperature are employed as ground materials having maximum particle size corresponding to ⅓ of the plastic packaging product wall thickness. The particle size lower than 10 micrometers usually meets the requirements of all the above mentioned applications.
Preferably, the anticorrosive plastic packaging materials of the invention are in the form of films or packaging products made of film or containers and/or porous emittors which are placed together with the article to be protected into another packaging product.
By the synergistic component of the system related to the present invention is constituted by the amorphous silicium oxide having mean particle size 1-20 micrometers the actual particle size is selected with respect to the packaging product wall thickness, and having pore volume in the range of 0,4-2,5 ml/g and pH value of 5% water suspension between 4,0 and 8,0.
For the given system the SiO2 grades having pore volumes between 1,2 and 1,8 ml/g and pH values of 5% water solution in the range of 5,0-8,0 (DIN ISO 787-9) are the most suitable.
All components are added to the material selected for the production of the plastic packaging product as dry homogenised mixtures or dry mixtures comprising polymer fluff However the most suitable application form is pelletised masterbatch of all components in the same type of polymer as it is used for plastic packaging product or in the polymer compatible with it.
Another possibility is to add mixture of pelletised masterbatch of all organic components and pelletised SiO2 masterbatch in polymers compatible with the plastic packaging material.
All above described procedures, providing that good homogeneity of all components in the packaging material is secured, guarantee that the packaging article will exhibit the same properties.
In case that multilayer packaging product is produced it is advantageous to add all components into the inner or middle layer by the procedures described earlier.
The basic advantages of the present synergistic system when compared with the use of the masterbatch of mixed metal anticorrosion inhibitors according to Czech application PV 1462-97v are the following: increased anticorrosive efficiency, decreased emissions during production, particularly of large surface area articles such as films, decreased exudation of additives to the article surface and consequently improved surface appearance and prolonged storage period of semi-final or final articles.